Wet cleaning using water is employed for wafer cleaning performed after various substrate processes. Japanese Patent Application Publication No. 2000-325894 (JP2000-325894A) describes a substrate cleaning apparatus including a rotation unit for rotating a wafer, a line-shaped ejection nozzle for ejecting cleaning liquid to a wafer in a line shape, and a moving unit for moving the line-shaped ejection nozzle along the wafer. In accordance with the substrate cleaning apparatus described in JP2000-325894A, even a large-sized wafer can be cleaned by cleaning water while ensuring high throughput and high cleaning performance.
Meanwhile, in order to prevent deterioration of device performance due to a microscale roughness of a wafer surface, a loss of a substrate material or the like, there is proposed a wet cleaning method using a diluted cleaning agent. In this wet cleaning method, deterioration of cleaning performance due to the dilution of the cleaning agent is compensated by a physical supplementary process. The physical supplementary process may include ultrasonic cleaning, two-fluid jet cleaning or the like. In the two-fluid jet cleaning, pure water and high pressure nitrogen gas are mixed and sprayed from a nozzle in a form of minute liquid droplets, and contaminants are removed by using shock waves generated by collision of the minute liquid droplets with the wafer surface.
However, the conventional wet cleaning method has the following technical problems caused by miniaturization and complication of device structures formed on a wafer.
First problem: When a physical supplementary process such as an ultrasonic wave, two-fluid spray or the like is used, a miniaturized device structure may be physically damaged.
Second problem: The miniaturized device structure may be collapsed by surface tension of water.
Third problem: When a substrate may be dried after wet cleaning, a watermark is generated on the surface of the substrate which leads to defects and deterioration of device performance.
Fourth problem: A water-soluble material formed on a substrate, e.g., lanthanum (La), may be damaged by a cleaning process.
Fifth problem: A substrate material, other than cleaning target objects, formed on a substrate may be lost by cleaning.
Sixth problem: A device structure may be broken by electric discharge generated between a substrate and water charged with electricity during a cleaning process.
As for a method for solving the above technical problems in the wet cleaning, there is suggested an ultra-low temperature aerosol ejection method using N2 gas and Ar gas. However, contaminants that can be cleaned are limited, and it is difficult to remove various contaminants.